The present invention relates to a needle actuation device for knitting machines particularly for hosiery or the like.
It is known that knitting machines for knitting hosiery or the like are generally constituted by a needle supporting element which can be substantially flat or cylindrical, depending on whether the machine has a rectilinear or circular model; the needle supporting element is faced by a cam supporting surface which supports a plurality of cams for actuating the needles during the operation of the machine.
More particularly, on the face of the needle supporting element that faces the cam supporting surface there is a plurality of mutually parallel slots, each of which accommodates a needle and optionally a needle pusher element. The needles and any needle pusher elements have heels which protrude from the needle supporting element toward the cam supporting surface and can engage in paths formed by the cams. The needle supporting element, during the operation of the machine, is actuated in one direction with respect to the cam supporting surface so that the heels of the needles or of the needle pusher elements engage in said paths, which have portions which are appropriately inclined with respect to the direction of motion in order to cause an alternating motion of the needles along the corresponding slots of the needle supporting element. This alternating motion causes the needles to take up the yarns fed at a feed of the machine and form loops of knitting.
The cam that determines the movement of the needle after said needle has taken up the yarn, at a feed of the machine, is termed casting-off cam because it produces a movement of the needle which forms a new loop of knitting by casting off the previously formed loop of knitting.
The profile of the casting-off cam can be engaged by the heels of the needles or of the needle pusher elements and is inclined with respect to the direction of motion of the needle supporting element with respect to the cam supporting surface indeed in order to achieve this movement of the needle.
In single-cylinder circular hosiery knitting machines, in which the needle supporting element is constituted by the needle cylinder, the casting-off cam, arranged directly after a feed or drop of the machine, causes the lowering of the needles inside the slots formed in the lateral surface of the needle cylinder after said needles have taken up the yarn at the feed or drop.
In many hosiery knitting machines, the casting-off cam is mounted on a slider which can move on command, with respect to the cam supporting surface, in a direction which is parallel to the extension of the slots that accommodates the needles, so as to allow a variation in the extent of the movement of the needles during the formation of new loops of knitting and thus allow a variation in the length of the loops of knitting.
In other machines, the casting-off cam is fixed to the cam supporting surface and the variation in the length of the loops of knitting is achieved by moving the needle supporting element with respect to the cam supporting surface parallel to the extension of the slots that contain the needles.
The profile of the casting-off cams currently used in hosiery knitting machines has a relatively high inclination with respect to the direction of motion of the needle supporting element relative to the cam supporting surface. The high inclination is necessary because during the formation of long loops of knitting the yarn must rest on a reduced number of sinkers or regions of the needle supporting element in order to reduce the friction that contrasts the sliding of the yarn during the formation of the new loops. If this friction is too high, it can lead to the breakage of the yarn or to a stretching of the loops formed previously in the same row of knitting. This high inclination, required in the formation of long loops of knitting, does not allow an increase in the operating speed of the machine, since it would lead to rapid wear of the casting-off cam and of the heels and might also cause the heels of the needles or of the needle pusher elements to break.